Igneous-electrolysis multicell furnaces,for the protection of the inner layer in contact with molten salts

ABSTRACT

DESCRIBED IS A FURNACE FOR THE ELECTROLYSIS IN MOLTEN BATH OF ELECTROLYTICALLY AGGRESIVE SALTS, IN PARTICULAR A MULTICELL FURNACE HAVING BIPOLAR ELECTRODES, FOR THE PRODUCTION OF ALUMINUM OR OF MAGNESIUM, HAVING AN LECTROLYSIS VAT THE BOTTOM AND THE WALLS WHEREOF COMPRISE A LAYER OF ELECTRICALLY CONDUCTING MATERIAL, IN PARTICULAR OR CAR BONACEOUS MATERIAL, INTERNALLY (THAT IS ON THE BATH SIDE) COATED WITH A LAYER OF REFRACTORY MATERIAL SUBSTANTIALLY INSULATING THE ELECTRICAL CURRENT. IN ORDER TO ELIMINATE OR REDUCE THE BYPASS OF ELECTRIC CURRENT THROUGH THE LAYER OF SAID REFRACTORY MATERIAL AND THE INCREASE OF CORROSION RESULTING THEREFROM ON SAID BOTTOM AND ON SAID WALLS BY THE MOLTEN MASS OF THE BATH PENETRATING INTO THE RESPECTIVE POROSITIES OR CRACKS, THE FURNACE IS CHARACTERIZED IN THAT THE VAT LAYER OR LAYERS, BEING BETTER ELECTRICAL CONDUCTORS, ARE MADE DISCONTINUOUS ALONG THE DIMENSION CORRESPONDING TO THE DIRECTION OF THE ELECTROLYSIS CURRENT, BY AT LEAST ONE SEPARATING BAFFLE, DIAPHRAGM OR TRANSVERSE LAYER MADE OF PRACTICALLY ELECTRONISULATING REFRACTORY MATERIAL THUS DIVIDING THE VAT INTO PORTIONS. THE TERMINAL ELECTRODES ARE ELECTRICALLY CONNECTED WITH THE CORRESPONDING VAT PORTIONS MADE OF CONDUCTING, E.G. CARBONACEOUS MATERIAL, BY MEANS OUTSIDE THE ELECTROLYTICAL BATH.

Feb. 9, 1971 Filed March 25, 1968 (3, D VARDA ETAL 3,562,136

IGNEOUS-ELECTROLYSIS MULTICELL FURNACES, FOR THE PROTECTION OF THE INNERLAYER IN CONTACT WITH MOLTEN SALTS 5 Sheets-Sheet 1 16 '18 25 20 22 2323 14 17 19\ 21 24 15 l J 1 1 7 7 '[fl l u 1 1 l u 26/ \1 a U Q 1 s 7 a9 10 11 12 1a z Z 24 n I r i r 24 L I I I I /A/k/ 4 .AA

I/l IL \T 3 4 14 17 9 38 19 31 21 I 23 2a 16 2a 1a 2032 as 22 15INVENTORS:

Quay e Mia 6L DE VAR ET AL 3,562,136

G. IGNEOUS-ELECTROLYS MULT LL FURNACES, FOR THE PROTE IO F TH NNER LAYERIN CO CT WITH I TEN SALTS Filed March 25, 1968 3 Sheets-Sheet 2 Feb. 9,1971 FIG. 2

Feb. 9, 1971 DE VARDA ETAL 3,562,136

' IGNEOUS-ELECTROLYSIS MULTICELL FURNACES, FOR THE PROTECTION OF THEINNER LAYER IN CONTACT WITH MOLTEN SALTS Filed March 25, 1968 3Sheets-Sheet 3 FIG.3

INVENTORS: um 7w 0U n Mr ac n u ml mm M M m m United States Patent US.Cl. 204243 10 Claims ABSTRACT OF THE DISCLOSURE Described is a furnacefor the electrolysis in molten bath of electrolytically aggressivesalts, in particular a multicell furnace having bipolar electrodes, forthe production of aluminum or of magnesium, having an electrolysis vatthe bottom and the walls whereof comprise a layer of electricallyconducting material, in particular of carbonaceous material, internally(that is on the bath side) coated with a layer of refractory materialsubstantially insulating the electrical current. In order to eliminateor reduce the bypass of electric current through the layer of saidrefractory material and the increase of corrosion resulting therefrom onsaid bottom and on said Walls by the molten mass of the bath penetratinginto the respective porosities or cracks, the furnace is characterizedin that the vat layer or layers, being better electrical conductors, aremade discontinuous along the dimension corresponding to the direction ofthe electrolysis current, by at least one separating baflle, diaphragmor transverse layer made of practically electroinsulating refractorymaterial thus dividing the vat into portions. The terminal electrodesare electrically connected with the corresponding vat portions made ofconducting, e.g. carbonaceous material, by means outside theelectrolytical bath.

Our invention relates to the vats for containing the molten bath inelectrolysis multicell furnaces used for instance in the production ofaluminum or magnesium. The invention is directed to prevention ofcorrosion of the refractory walls in direct contact with the molten bathand/ or molten metal.

More particularly, our invention relates to the vats of electrolysismulticell furnaces having suspended electrodes, internally coated with alayer of a special refractory material being a poor electrical currentconductor. This special refractory material may, for instance, consistof silicon nitride bonded silicon carbide, e.g. the commercial productsRefrax and Christolon. Backing said special refractory material is alayer of carbonaceous material having much higher electric conductivitythan the special refractory material. This carbonaceous material layeris to secure the vat tightness, i.e. to prevent leakage from the vat ofthe molten bath and/or molten metal which might seep through either thepores of said special material or through the joints or any crackstherein.

In these furnaces, going outwardly from the layer of carbonaceousmaterial, are one or more layers of heatinsulating material, all ofwhich is contained in an iron shell. It is known that the thickness ofthe individual vat layers of carbon or of heat-insulating materials iscalculated by taking into account heat dispersion. In conventionalfurnace vats, the freezing isotherm surface of the molten bath must, ifpossible, be contained within a zone which for practical purposes issufiiciently immune against destruction by the action of the seepedbath. No serious inconveniences are met, if and until said freezingsurface remains wholly contained within the layer of carbonaceousmaterial. That means, one achieves what is technically called a thermalblock. This is a blocking by freezing 5 the advance of the infiltration,when molten bath arrives at a vat zone with an isotherm corresponding tothe freezing temperature of the bath.

In the multicell furnaces having suspended electrodes, operating with anelectrolysis bath composed of molten salts, such as for instancecryolite baths in the electrolysis of aluminum oxide, for the productionof metallic aluminum, the following two problems, amongst others, giverise to serious drawbacks and surprisingly are resolved or greatlyattenuated by the present invention:

The first problem relates to the protection of the refractory layer indirect contact with the molten bath. The second problem relates to thepossibility of short-circuiting the operation of a multicell furnacehaving suspended electrode. In these furnaces a considerable part of theelectrical current feeding the furnace passes as parasitic currentthrough the vat walls as follows: terminal anodebath-refractory layerinternally coating the vat-carbon vat (zone close to the terminalanode)carbon vat (zone close to the terminal cathode)-refractorymaterial-bath-terminal cathode.

It is known that certain special refractory materials resist quite wellthe action of molten salts, for instance molten cryolite at 900960 C.However, in an electrolysis furnace where the chemical action isaccompanied by an electrochemical action, there occurs, especially inmulticell furnaces, a deep attack of the refractory material by more orless intense corrosion, up to the disgregation or complete destructionof the special refractory material in the zones more exposed to theelectrochemical action of the electrical current. The higher thetemeprature, the more rapidly these deleterious phenomena occur. Thechemical composition of the bath may contribute to aggravating thisgreat inconvenience.

In what follows, reference will be made in particular to multicellelectrolysis furnaces having suspended electrodes, for the production ofaluminum, but without limitation thereto of the present invention, sinceit can be applied to all the multicell furnaces presenting analogousconditions regardless of the electrolytical molten bath and the metalproduced.

FIGS. 1 to 3 of the drawing show schematically in plan view and insections along lines AA and BB, respectively, an embodiment according tothe present invention, of a multicell furnace having suspendedelectrodes, with seven cells, for the electrolysis of aluminum oxide.

The special refractory material in contact with molten bath, under theoperating conditions, soaks up fluoridecontaining bath and becomeselectrically conductive. Although the electrical resistivity of the thussoaked refractory materialis considerably higher than the resistivity ofthe electrolytical bath, a part of the electrolysis current is shuntedby the internal vat coating consisting of special refractory material.

This shunting phenomenon is greatly aggravated by the presence of thecarbon layer backing the special refractory material, as carbon is agood electrical conductor. The coating of special refractory materialwhich should act as an insulating screen fails whereby a strongparasitic current passes through the refractory layer into the carbonlayer partially attacking and destroying those zones of specialrefractory material which are next to the bipolar electrodes. Thosezones of special refractory material next to the terminal electrodes,and particularly to the terminal cathode, are more strongly attacked.The latter zone of the vat, in fact, will thus function as anode. Thezones where the electrical current enters and comes out, passing fromthe bath into the refractory material or from the refractory materialinto the bath, are subject to localized, often deep attacks of therefractory material itself. A remarkable part of the electrolysiscurrent introduced into the multicell furnace is thus wasted throughthis parasitic shunt, i.e., through the vat layers made of specialrefractory material and of carbon.

Of course, the portion of current shunted through the special refractorymaterial and the carbon of the vat increases as the distance between thewalls of refractory material and the terminal electrodes becomes smallerand the number of the cells and the amperage with which the furnace isfed becomes greater.

The corrosion effects grow by geometrical progression with the increaseof intensity of the thus shunted current. When examining the fiow of theabove parasitic current, one can see that also this current passes atleast twice through the cryolitic molten bath, giving rise to theformation of parasitic secondary cells which, while perhaps producingaluminum, do so with local destructive effects on the vat of refractorymaterial, particularly in the zones on the faces of the specialrefractory material, which are next to the terminal electrodes,especially next to the terminal cathode. Moreover, per unit consumptionof electrical energy becomes high, due to the fact that a considerablepart of the electrolysis current is shunted, as above described, insteadof regularly flowing through the row of the single cells of the muticellfurnace.

We have found that it is possible to eliminate either completely or atleast substantially the above drawbacks, by very simple expedients whichcan be easily achieved. More precisely and surprisingly, we use acombination of expedients which apparently would have opposite anddestructive effects.

According to our invention, the carbon vat is divided into slices bytransverse separating layers of refractory material which interrupt theelectrical continuity between the carbonaceous material at the back ofthe special refractory material. The effect of such division can beimproved in that the slices of carbon vat resulting thereby, or at leastthose corresponding, by their position, to the terminal electrodes, areconnected outside the bath to the electrodes by good external electricalconductors which we shall call external shunts as used hereinbelow.

In a preferred embodiment, the number of the carbon slices correspondsto that of the electrodes, or of the cells, and the external shunt isprovided not only for the two terminal electrodes but also for theintermediate bipolar electrodes. Combining these two features, whichwould appear to increase at least the causes of eddy current losses,results, however, in a stopping blockage of the corrosion on the innerface of the refractory coating, on the contrary, as well as a reduction.of the current loss through the vat. The external shunt, in fact,provides equipotentiality of the electrodes and vat slices incorrespondence with each cell.

In this way, we almost completely eliminate the parasitic electricalcurrent which formerly entered from the bath into the special refractorymaterial next to the terminal anode and left the special refractorymaterial towards the bath in the zones next to the terminal cathode.Moreover, this modest current which will continue to pass through thevat (from the vat zones next to the terminal anode, to the vat zonesnext to the terminal cathode), will no longer enter into or come outfrom the special refractory layer on the bath side, but on the carbonside. It is thus possible to eliminate conspicuous phenomena ofdestructive corrosion, which the special refractory material formerlyunderwent in the multicell furnaces.

In multicell furnaces of the kind to which the present invention refers,collecting pits for the molten aluminum produced by the electrolysis areprovided on the vat bottom, preferably in correspondence with thevarious cells or, better, in the interelectrodic spaces. These pits,depending on the nature of the electrolytical process, may be completelyindependent and constructed in a way that the separating sectors contactthe lower part of the respective overlying anodes-cathodes. In the caseof the electrolysis for aluminum, the pits may be at a certain distancefrom the anodes or cathodes, e.g., 35 cm., so as to allow the freecirculation of the bath between one pit and the next. The pits can beformed by small walls or partitions made of special refractory material,and in particular can be connected with or be a part of the aforesaidseparating layers (partitions) or baffles (diaphragms) of specialrefractory material which establish the discontinuity of the carbon vat,according to the present invention.

The material of the bafiles or separating layers can be the same as thatof the inner coating, or may be different.

, In the latter case, it must be kept in mind that the material of thebaffles (diaphragms) or separating layers should be a better electricalinsulating material than the inner coating (special refractory materialas defined above) of the vat, whereas it may be less resistant to attackof the bath and less resistant to heat. For instance, electrocastalumina can be used.

Therefore, when using our invention, the electrolysis current enteringthe furnace through the current-carrying studs of the terminal electrodewhich functions as anode, and exiting from the current-carrying studswhich are connected to the multicell furnace on the side of the terminalcathode, is shunted only partially through the refractory vat, which iscoated externally with a layer of discontinuous carbonaceous material,i.e. subdivided by the said partitions made of refractory material. Thecurrent shunted through the conductors outside the bath enters therefractory material of the walls; partially soaked with bath andtherefore, in turn, to a certain extent a conductor of the electrolysiscurrent; on the side opposite to that of the bath, following a coarsewhich goes from the anodic zone, i.e. from the terminal anode, to thecathodic zone, i.e. to the terminal cathode, and comes out from therefractory material through the adjacent layer of carbonaceous materialand not through the bath itself.

Moreover, owing to the resistivity of the refractory wall which isimpregnated with bath, which resistivity is a multiple of the electricalresistivity of the bath alone, the thus shunted or parasitic currentrepresents only a small fraction of that fed to the furnace, whereasmost of the current, when passing through the cells of the furnace,performs the electrolysis of the aluminum.

Having set forth its general nature, the invention will be bestunderstood from the more detailed description hereinafter which refersto the accompanying drawings. Although the drawings illustrate onearrangement of apparatus in which the process of this invention may bepracticed, it is not intended to limit the invention to the particularapparatus or material described.

EXAMPLE In a furance with seven cells and 5 ka., built according to thepresent invention, the current shunted through the layers of refractorymaterial (which in this specific case was Refrax) and of carbon, was ofthe order of 5% on the furnace feeding current. The Refrax refractorymaterials in contact with the molten bath, after several months ofoperation, were found, on dismantling the furnace, to have been attackedon the surface only to a depth of from 1 to 2 mm.

In all the figures, the external iron shell 1 encloses the layers ofinsulating material 2, of carbonaceous material 3, of special refractorymaterial 4. All these layers form the vat of the multicell furnace. Thisvat is filled with molten cryolite bath 5, in which are suspended theelectrodes of carbonaceous material 6, 7, 8, 9, 10, 11, 12 and 13, by adevice known per se and not shown, from the top and immersed in thebath. The electrolysis current enters through proper current-carryingstuds 14 into the terminal anode 6 and after having run through the rowof the seven cells, comes out from the terminal cathode 13 throughcurrent-carrying studs 15. The layer of refractory material 4; and oftenalso the part, next to the bath, or baflles 16, 17, 18, 19, 20, 21 and22, made of refractory material, too; is impregnated with bath, sincealso the special refractory materials usually have a certain degree ofporosity. In the carbonaceous Zones, separated from each other bypartitions 16, 17, 18, 19, 20, 21 and 22, are fixed iron studs 23protruding upwards, connected by flexible electrical conductors 24 withthe studs fixed on electrodes 6, 7, 8, 9, 10, 11, 12 and 13. These studsmay coincide, as in the drawing, with the current-carrying studs 14 and15 for the terminal electrodes 6 and 13, respec tively. The studs of theintermediate electrodes are generically indicated by 25.

The external conductors 24; which electrically shortcircuit the carbonzone 26 of the anodic head of the vat with the terminal anode 6, andelectrically short-circuit the carbon zone 27 of the cathodic head ofthe vat with the cathode 13, and analogously, optionally short-circuitelectrically also the intermediate electrodes 7, 8, 9, 10, 11, 12, withthe corresponding zones or intermediate carbon slices 28, 29, 30, 31,32, 33, respectively; can be connected with the electrodes by means ofstuds of the same electrodes. They also may be the current-carryingstuds for the terminal electrodes or can be either individual studs, orstuds acting simultaneously as suspension organs of the same electrodes,on one side, and corresponding studs provided on the carbonaceous zonesor slices of the vat, on the other side, preferably on both longitudinalsides of the furnace.

The seven-cell furnace illustrated in the figures has eight zones ofcarbonaceous material which sheathe the outside of as many zones of thespecial-refractory layer in contact with the molten bath as correspondto the eight electrodes suspended in the same bath.

The wall partitions 34 divide the furnace bottom into pits 35, for thecollection of the produced aluminum.

The present invention makes it possible to direct the freezing isothermof the bath to a zone consisting of carbonaceous materials of low costand practically immune against destruction by action of the infiltratedbath. Such a zone of sufficient width is created by renderingequipotential the electrodes and the corresponding vat portions,controlling at, the same time the current losses through the vat, andthereby also the formation of Joule heat generated by said current inthe carbon, thanks to the transversal partitions made of refractorymaterial.

Under the scope of the present invention fall the obvious variants andthe obvious constructive and functional equivalents, either for instanceas concerns the number and the position of the separating bafiies(diaphragms) of refractory material, or for instance as regards thechoice of the special refractory material or its substitution by anothermaterial, or of the means for giving the electrodes and vat portions thesame electric potential. Thus, for instance, there may be one or moreintermediate electrodes, or one or more intermediate vat portions, notconnected in this Way.

We claim:

1. Furnace for the electrolysis in molten bath of electrolyticallyaggressive salts, said furnace being a multicell furnace having bipolarelectrodes, having an electrolysis vat with the bottom and the wallscomprised of a layer of electrically conducting material, coated on thebath side or inside with a layer of refractory material substantiallyinsulating the electrical current, wherein the vat layer, being a betterelectrical conductor, is made discontinuous along the dimensioncorresponding to the direction of the electrolysis current, by at leastone separating ballle made of practically electroinsulating refractorymaterial dividing the vat into portions.

2. The furnace of claim 1, wherein the terminal electrodes areelectrically connected with the corresponding vat portions made ofconducting material, by means which are outside the electrolytical bath,whereby the passage of electric current through the layer of saidrefractory material and the increase of corrosion resulting therefrom onsaid'bottom and on said walls by the molten mass of the bath penetratinginto the respective porosities or cracks are at least substantiallyreduced.

3. The furnace of claim 1, wherein the layer of practically insulatingrefractory material internally coating the vat is made of siliconnitride bonded silicon carbide.

4. The furnace of claim 1, wherein the practically electroinsulatingrefractory material of said separating baflles is silicon nitride bondedsilicon carbide.

5. The furnace of claim 1, wherein the electrically conducting materialof the vat is carbonaceous.

6. The furnace of claim 1, wherein the practically electroinsulatingrefractory material of said separating baffies is electrocast A1 0 7.The furnace of claim 1, wherein the vat portions correspondapproximately with the electrodes.

8. The furnace of claim 2, wherein at least one intermediate electrodeis connected with vat portions.

9. Multicell furnace accordingto claim 1, wherein the separating bafflesof refractory material constitute, on the vat bottom, separating wallsor partitions either forming or delimiting pits on the vat bottom, incorrespondence with the single electrolysis cells of the multicellfurnace, to collect the metal produced in said cells.

10. The furnace of claim 2, wherein the electrically connecting meansoutside the bath are flexible connectors attached to studs provided onthe vat portions made of conducting material and, respectively, to theelectrodes.

References Cited UNITED STATES PATENTS 3,321,392 5/1967 McMinn et a1204-243 JOHN H. MACK, Primary Examiner D. R. VALENTINE, AssistantExaminer US. Cl. X.R. 204-244

